This invention relates generally to integrated circuit memories and more specifically to non-volatile memories.
Semiconductor memories are widely used in computer systems and come in many varieties. Non-volatile memories represent one important type of memory. These memories continue to store information even if power to the memory is turned off or otherwise interrupted.
Ferroelectric material has recently been used to make non-volatile memories. These non-volatile memories typically contain an array of cells containing ferroelectric capacitors. These cells are accessed by a grid of row and column lines since each cell is connected between one row and one column line. By applying a voltage between one row and one column line, one cell is uniquely addressed and the applied voltage appears across the addressed cell.
The addressed ferroelectric cell acts like a capacitor and charges up to the applied voltage. Unlike a capacitor, though, the cell keeps it charge even after the applied voltage is removed. In this way, the memory can store information when no voltage is applied to the memory. To store a logic one, a cell is charged to a first voltage. To store a logic zero, a cell is charged to a second voltage. The second voltage usually has the same magnitude as the first voltage but is of opposite polarity.
Information is read from a cell in a destructive read operation. As part of the read operation, voltage which is sufficient to store a logic one in a cell is applied to the cell. The amount of current flowing through the cell while this voltage is applied indicates the value previously stored in the cell. A relatively high current flow indicates the cell changed from storing a logic zero to storing a logic one. A relatively low level of current flow indicates the cell previously stored a logic one. Thus, the state of the cell is sensed by measuring the current flow when the reading voltage is applied.
After the sense portion of the read operation, the cell will always store a logic one. If the cell contained a logic zero which is changed to a logic one during a read, the previous value stored in the cell must be restored. To achieve this result, logic circuitry built into the memory initiates a write operation to write a logic zero into the accessed cell.
Destructive read out suffers from two shortcomings. First, it can take up to two access cycles to read information from memory. In the first, the information is sensed. In the second cycle, the information is rewritten into the cell. This extra cycle can slow down operation of a ferroelectric memory.
A second shortcoming of a destructive readout is that the information in memory may be totally lost if the power to the memory is removed during the read operation. In particular, if power is lost after the contents of the cell are sensed but before the contents are rewritten, correct contents of the cell will never be restored. Since non-volatile memory is often used to store information which must be saved in the event of an unexpected power loss, this second shortcoming can present a significant problem.